This invention relates to the production of polymers in fluidized beds, particularly in fluidized bed processes for the polymerization of olefins, adjusted to operate turbulently to facilitate high levels of liquid in the recycled fluid.
The production of polyolefins in fluidized beds requires that the heat of reaction be removed in order to maintain appropriate temperatures for the desired reaction rate. In addition, the temperature of the vessel cannot be permitted to increase to the point where the product particles become sticky and adhere to each other. The heat of reaction is commonly removed by circulating the gas from the fluidized bed to a heat exchanger outside the reactor and passing it back to the reactor.
The earliest such recycle systems were based on the assumption that it would be inefficient, or inoperable, to cool the recirculating gas below its dew point so that liquid would be introduced into the reactor through the recycle process. However, operation in the xe2x80x9ccondensing modexe2x80x9d has become quite common in the artxe2x80x94see Jenkins U.S. Pat. Nos. 4,543,399 and 4,588,790. In accordance with the teachings of these patents, an inert liquid may be introduced into the recycle stream to increase its dew point. The resulting ability to remove greater quantities of heat energy in less time has increased the production capacity of the typical exothermic fluidized bed reactor.
More recently, in U.S. Pat. Nos. 5,352,749, 5,405,922, and 5,436,304 (see column 12, lines 4-17), higher levels of liquid have been shown to be practical. Griffin et al, in U.S. Pat. No. 5,462,999, observe a range of bulk density functions Z, which include dependence on temperature, pressure, particle characteristics and gas characteristics. As in the Griffin et al ""999 patent, which is incorporated by reference, we refer herein to the bulk density function Z, defined (col. 12, lines 38-47 of Griffin ""999 and col. 12, lines 31-42 of Griffin et al U.S. Pat. No. 5,436,304) as   Z  =                    (                              p            bf                    -                      p            g                          )            /              p        bs                            (                              p            s                    -                      p            g                          )            /              p        s            
where pbf is the fluidized bulk density, pbs is the settled bulk density, pg is the gas density and ps is the solid (resin) density. The bulk density function Z can be calculated from process and product measurements.
Fluidized bulk density (FBD), and particularly the ratio of fluidized bulk density to settled bulk density (SBD), are asserted to be limiting factors for stable operation where higher quantities of liquid are used in the recycle stream. DeChellis and Griffin, in U.S. Pat. No. 5,352,749 (also incorporated in its entirety by reference) place an upper limit of 5.0 feet per second (1.5 m/sec) on the superficial gas velocity (xe2x80x9cSGVxe2x80x9d) within the reactorxe2x80x94see column 8, lines 31-33. The various perceived limits on operating conditions have inhibited workers in the art from increasing the level of liquid in the recycle stream and from venturing into the realm of turbulence in the fluidized bed. DeChellis and Griffin, in U.S. Pat. No. 5,352,749, maintain the ratio of FBD/SBD above 0.59 (col. 4, line 68), stating xe2x80x9cas a general rule a reduction in the ratio of FBD to SBD to less than 0.59 may involve risk of fluidized bed disruption and is to be avoided.xe2x80x9d (Col. 5, lines 10-12).
Govoni et al, in U.S. Pat. No. 5,698,642 (col. 2, line 40), refer to the xe2x80x9cturbulencexe2x80x9d generated by the grid (distributor plate) which distributes the liquid into the bed of polymer in the DeChelllis et al ""749 patent, but this is not turbulence as defined (see below) in turbulent fluidization. Unlike the present invention, Govoni et al operate under fast fluidization conditions.
Definition of the Turbulent Regime
There are at least five different fluidization regimes. In order of increasing gas velocity (U) or decreasing solids concentration, they are particulate fluidization (for group A particles only), bubbling fluidization, turbulent fluidization, fast fluidization, and pneumatic transport. Gupta and Berruti also describe xe2x80x9cdense phase conveying,xe2x80x9d a fluidization regime that qualitatively can be considered an extension of the turbulent regime where there is no dilute freeboard above the bed as is common in olefin polymerization, resulting in high solids carryover at the top of the fluid bed reactor. Gupta and Burruti, Fluidization IX, 1998, p. 205. We include dense phase conveying in the definition of turbulent fluidization for purposes of our invention.
A turbulent regime is not simply a regular dense bed of bubbling fluidization regime having substantial freeboard activities. The turbulent regime has distinct features differing from those of the bubbling and fast fluidization regimes. Most available models and correlations developed for bubbling fluidization regimes or fast fluidization regimes cannot be applied for turbulent fluidization regimes.
The mean amplitude of pressure fluctuations in the fluidized bed has been observed as having a noticeable downturn as the superficial gas velocity increased to a certain point. The peak mean amplitude fluctuation was taken as the velocity for the beginning of a transition to turbulent fluidization, and denoted Uc. See Lee, G. S. and Kim, S. D., Journal Chemical Engineering (Japan) vol. 21, No. 5 (1988), 515. Uc is defined as the velocity at which amplitude of pressure fluctuations peak. We note that it marks the transition from the bubbling regime to the turbulent regime, and accordingly we sometimes call it herein the transition velocity. In addition to the amplitude of pressure fluctuations, characteristic indicia of pressure fluctuation intervals, standard deviation of pressure fluctuation, skewness and flatness of pressure fluctuations, and power spectral density function of pressure fluctuations may also be observed at Uc according to Lee and Kim. However, their correlation of the Archimedes Number to the critical Reynolds Number for turbulence is not applicable to pressurized fluid bed polymerization. The velocity at which the mean amplitude of pressure fluctuations level off as the gas velocity is increased beyond Uc is defined as Uk, as will be illustrated herein in FIG. 3. We take the appearance of Uk as marking the termination of turbulent fluidization and the onset of fast fluidization, as the superficial gas velocity increases.
The structure of a fluidized bed changes when the gas velocity exceeds Uc. The most important difference is in the bubble behavior. Specifically, the bubble interaction is dominated by bubble coalescence at gas velocities smaller than Uc, while it is dominated by bubble break-up at gas velocities greater than Uc (e.g., Cai et al., xe2x80x9cEffect of Operating Temperature and Pressure on the Transition from Bubbling to Turbulent Fluidizationxe2x80x9d, AIChE Symposium Seriesxe2x80x94Fluidization and Fluid Particle Systemsxe2x80x94Fundamentals and Application, No. 270, v. 85 page 37, 1989; Characterization of the Flow Transition between Bubbling and Turbulent Fluidizationxe2x80x9d by Ahmed Chehbouni, Jamal Chaouki, Crisstopher Guy, and Danilo Klvana, Ind. Eng. Chem. Res. 1994, 33, 1889-1896. The bubble/void size in the turbulent regime tends to decrease with the increase of gas velocity due to the predominance of bubble break-up over bubble coalescence. This trend is opposite to that in the bubbling regime. Thus, with sufficiently high gas velocity, bubble/void size can be reduced to an order of magnitude similar to the particle size. This high gas velocity, called the transition velocity, demarcates the diminishing of bubbles in the turbulent regime and a gradual transition to lean-phase bubble-free fluidization. As a result of the dominant break-up tendency of bubbles/voids, more small bubbles/voids with relatively low rise velocities and longer residence time exist in turbulent systems, which leads to a more significant dense bed expansion than that in the bubbling regime, and therefore a lower fluid bed density. Bubbles/voids in the turbulent regime are less regular in shape compared with those in bubbling beds. At relatively high gas velocities in the turbulent regime, the clear boundary of bubbles/voids disappears and the non-uniformity of solids concentration distribution yields gas voids which become less distinguishable as the gas velocity further increases towards fast fluidization.
Govoni et al, in U.S. Pat. No. 5,698,642, define xe2x80x9cfast fluidizationxe2x80x9d as the state obtained xe2x80x9cwhen the velocity of the fluidizing gas is higher than the transport velocity, and it is characterized in that the pressure gradient along the direction of transport is a monotonic function of the quantity of injected solid, for equal flow rate and density of the fluidizing gas.xe2x80x9d The patent continues (column 5, lines 20-30) xe2x80x9cContrary to the present invention, in the fluidized-bed technology of the known state of the art, the fluidizing-gas velocity is maintained well below the transport velocity, in order to avoid phenomena of solids entrainment and particle carryover. The terms transport velocity and fast fluidization are well known in the art.xe2x80x9d
Some features of the turbulent regime different from those of a bubbling regime are as follows:
i) Bubbles/voids still exist, with a predominant tendency of break-up. Their sizes are small and decrease with the increase of gas velocity. Bubbles split and reorganize frequently, and often appear in more irregular shapes.
ii) Bubbles/voids move violently, rendering it difficult to distinguish the emulsion (continuous) and bubble/void (discrete) phases in the bed.
iii) Dense phase expands significantly with an expansion ratio (ratio of fluidized bed height to still bed height) greater than that of a bubbling regime. The upper surface of the bed exists, but becomes more diffused with large particle concentration in the freeboard.
iv) Bubble motion appears to be more random with enhanced interphase exchange and hence intimate gas-solid contact and high beat and mass transfer.
Persons conversant with the art of fluidized beds started to accept a more or less refined definition of the xe2x80x9cturbulentxe2x80x9d fluidization regime around the mid-1980""s. This evolved definition recognizes the xe2x80x9cturbulentxe2x80x9d fluidization regime as a unique operation range which starts at about U greater than Uc and covers at least a major part of Uc less than U less than Uk. Because of its substantial structural difference from bubbling regime and intensive application background, this definition for xe2x80x9cturbulentxe2x80x9d regime has gained substantial acceptance in the world fluidization community. Nevertheless, at least until recently, workers in the art have not been particularly consistent or precise in their use of the term xe2x80x9cturbulent fluidizationxe2x80x9d in a fluidized bed. See the critique of a number of other papers: xe2x80x9cWhat is Turbulent Fluidizationxe2x80x9d by Martin Rhodes, Powder Technology 88 (1996) 3-14. However, it is now generally accepted that turbulence is achieved when a significant portion of the bubbles begin to lose their shape near the top of the bed, and a turbulent motion of clusters and voids of gas of various sizes and shapes appears. As the superficial gas velocity is increased, the onset of turbulence is associated with a critical superficial gas velocity, commonly called the transition velocity. See Cai et al, supra. The authors provide a plot of the mean amplitude of pressure fluctuations against the gas velocity in fluidized beds, showing the bubbling regime and the turbulent regime clearly separated by the xe2x80x9ccritical superficial gas velocity,xe2x80x9d Uc. The critical superficial gas velocity appears at a peak; as the plot proceeds into the turbulent regime, the mean amplitude of pressure fluctuations recedes. See also Chehbouni et al, supra. On the other end of the regime, turbulence yields to fast fluidization when the bubbles and voids are diminished in size to the same order of magnitude of the solid particles. See Avidan, U.S. Pat. Nos. 4,547,616 and 4,746,762, and Kushnerick et al U.S. Pat. No. 4,827,069, all incorporated herein by reference because of their description of turbulence in fluidized beds. Thus we use xe2x80x9cturbulencexe2x80x9d, xe2x80x9cturbulentxe2x80x9d, and xe2x80x9cturbulent fluidizationxe2x80x9d to mean the state of a fluidized bed existing between the conditions of (1) the presence of discernable bubbles and (2) fast fluidization, and/or the regime of conditions between (a) the transition velocity Uc and (b) the transport velocity Uk, expressed as the superficial gas velocityxe2x80x94see the Avidan U.S. Pat. No. 4,746,762 patent at column 7, lines 65-68, for example.
It is recognized that turbulent fluidization might not exist homogeneously across the vertical dimension of the bed. Turbulent fluidization may begin at the top of the bed and move progressively lower as the superficial gas velocity increases. It is thought that turbulent fluidization aids in the mixing of liquid and polymer particles in the region of the bed near the distributor plate, and the presence of turbulent fluidization at or near the distributor plate is therefore preferred to other isolated areas of turbulence. For our purposes in this invention, turbulent fluidization is meant to include a zone of turbulent fluidization in the bottom, middle or top of the fluid bed, as well as a turbulent regime throughout the bed as described above.
Our invention is a method of achieving and utilizing a high percentage of liquid in the recycle in order to remove heat from the recycle at a faster rate, thus enabling a faster production rate. We achieve a high percentage of liquid in the recycle by deliberately adjusting the conditions in the reactor to pass from the bubbling mode of fluidization to turbulent fluidization and increasing the condensing level (the amount of liquid introduced through recycle) to a desired level of 17.5% or higher, preferably 20% or higher, as will be explained further herein. We maintain operation in the turbulent regimexe2x80x94that is, we do not increase the gas velocity to Uk or beyond.
Although, as indicated above, it is accepted and valid to define turbulence as a regime between (1) that in which there are discernable bubbles, the bubbling regime, and (2) fast fluidization, we believe a more precise and objective definition is the regime between Uc and Uk as explained above. While our invention is useful and operable throughout the entire range between discernable bubbles and fast fluidization, our preferred regime is that between a superficial gas velocity (SGV) of 1.01xc3x97Uc and the onset of fast fluidization, or Uk, and the most preferred regime, or range of operation, is that between 1.1xc3x97Uc and 0.9xc3x97Uk.
Preferably, we utilize a ratio of fluidized bulk density to settled bulk density (FBD/SBD) less than 0.59, more preferably in the range of 0.2:1 to 0.58:1, most preferably in the range 0.4 to 0.55, together with a high percentage of recycle liquidxe2x80x94that is, at least 17.5% by weightxe2x80x94preferably 20% to 90%, more preferably 20% to 50%. Our low fluidized bulk density results in a low ratio of fluidized bulk density to settled bulk density. We are able to use a low fluidized bulk density together with a high liquid recycle rate because we operate in the turbulent condensing mode. While the defining characteristic of the turbulent mode we use is the range of SGV described above, we also prefer that the FBD/SBD ratio be maintained less than 0.59:1, preferably 0.4 to 0.58.